Circuit breaker for electric instruments



Se t. 18, 1956 L. J. ANDRES CIRCUIT BREAKER FOR ELECTRIC INSTRUMENTS Filed Oct. 2, 1953 United States Patent (like CIRCUIT BREAKER FOR ELECTRIC INSTRUMENTS Lloyd J. Andres, Chicago, Ill., assignor to Permo, Incorporated, Chicago, 11]., a corporation Application October 2, 1953, Serial No. 383,856

3 Claims. (Cl. 200-87) This invention in general relates to electric circuit breakers and more particularly to a sub-miniature circuit breaker for use in and for protecting electric indicating instruments from damage due to overload.

Electric indicating instruments of the sensitive low current variety cannot be readily protected against burnout or damage due to overload because the current requirement for their full scale reading is usually extremely low, in many instances as low as 50 microamperes. Fusible elements are not generally applicable because of their lack of sensitivity and electronic protecting devices are costly and complicated.

The circuit breaker hereinafter described provides a complete solution to the problem of protecting sensitive instruments by the provision of a sensitive normally closed switch actuated by the instrument pointer when moved by overload beyond its normal scale reading.

A principal object of the invention is the provision of a circuit breaker, having self-contained adjustments, which is applicable to installation in a wide range of ammeters and voltmeters and capable of being reset by the manual rotation of the existing pointer zero-set adjustment screws on the instruments.

Another object of the invention is the provision of a circuit breaker for electric instruments which include a movable switch blade positioned in the path of movement of the instrument pointer when said pointer is moved beyond its normal range including means for holding said switch blade in displaced position and including additional means for adjusting the sensitivity of said blade.

These and other objects and advantages in one embodiment of the invention are described and shown in the following specification and drawing in which,

Fig. 1 illustrates a typical electric indicating instrument in which left and right circuit breakers are mounted on the scale plate thereof;

Fig. 2 is an enlarged plan View of the left circuit breaker shown Fig. 1, with the upper portion of the body thereof broken away;

Fig. 3 is a side view of the circuit breaker shown in Figs. 1 and 2 viewed from a position occupied by the pointer in its normal rest position;

Fig. 4 is an enlarged cross sectional view taken through section line 44, Fig. 2.

Referring to Fig. 1, a front view of a typical electrical instrument 1 is shown having both left and right circuit breakers secured to a scale plate thereof and positioned for actuation in the path of overloaded movement of the pointer 2 in either direction. The left circuit breaker is shown in its normal position and the right circuit breaker is shown in its tripped position. A body 3 made of dielectric material, such as Bakelite, contains and supports all elements of each circuit breaker. In the embodiment shown, the right and left circuit breakers are mirror images of each other. This construction lends itself for easy and simple installation in existing instruments upon the scale plate of the instrument in the space between the plate and the inside surface of the window.

2,763,748 Patented Sept. 18, 1956 Referring to Figs. 1 and 2, each body is secured to the upper surface of the instrument scale plate by countersunk screw 4 and nut 5. A dowel 6 integral with the body 3 projects through an appropriate hole in the scale plate to position the body in predetermined relation with the path of movement of the pointer 2. Thus it is apparent that the provision of two holes in the scale plate is all that is required to mount each circuit breaker in the instrument.

Referring to Fig. 2 a switch blade 7 is secured at one end to the body 3 by a screw 8 threaded into body 3 with a solder terminal 9 serving as a washer. The blade 7 is made of paramagnetic material, preferably spring steel, and plated with high conductivity metal, such as silver or gold. It is important that the blade be made of exceedingly thin material in the order of .0015 inches to accomplish the desired flexure sensitivity and be of exceedingly low mass to resist unwanted fiexure when the instrument is subject to shock. The contact portion 10 of the spring blade is adapted to make and break contact with the undersurface of the head of screw contact 11 which is adjustably threaded into the body 3 as shown. The screw contact is made of low resistance metal or plated like the blade 7. A U-shape abutment 12 integral with the blade 7 is positioned in the path of the instrument pointer 2 and adapted to be moved thereby.

A tension adjustment screw 13 is threaded into body 3 having one end bearing against blade 7 for the purpose of adjusting the tension in blade 7 as required to suit various operating conditions to be hereinafter described. A cylindrical permanent magnet 14 and a pair of pole pieces 15, polarized north and south respectively, are secured in the body 3 as shown with the outer ends of the pole pieces positioned to attract and temporarily retain the blade against their outer surfaces when the blade is displaced. The width of the air gap 16 has an important bearing on the operation of the circuit breaker since the Zone of magnetic force exerted by the pole pieces should exert a minimum pull on the blade when the blade is in its normal position as shown. The extent of the external magnetic field about the ends of the pole pieces is a function of the width of the air gap, hence the Width of the air gap must be adjusted with respect to the normal position of the blade for a proper tripping action of given sensitivity. The magnetic force should not materially influence the blade until the latter is partially displaced toward the pole pieces a predetermined distance. The magnetic force exerted by the pole pieces are intended to assist the movement of the blade to its tripped position against the pole pieces, as shown in dotted lines.

Referring to Figs. 2, 3 and 4, a reset lever 17 is pivotly secured for limited movement to body 3 by clevis pin 18 as shown. A bumper 19, made of insulating material, is secured to the offset end 2% of the lever 17 and adapted for movement through a clearance hole 21 in the body 3. The opposite end of the lever 17 terminates in a tab 22 for camming engagement against the zero-set lever 23. The zero-set lever shown is a conventional type used in most electric instruments and is adapted to be oscillated through a predetermined angle by an eccentric pin, not shown, by the Zero-set screw 24 for the purpose of adjusting the instrument pointer to its zero position.

A base plate 25, preferably made of thin metal, best shown Figs. 3 and 4, is retained between the body 3 and the upper surface of the instrument scale plate for the purpose of providing a stop abutment 26 for the reset lever and a colored signal flag 27 for quick observation when uncovered by the U-shaped abutment 12 of the switch blade 7.

A coil spring 28 retained in a bore in body 3 coaxial with screw 8 normally urges the reset lever 17 in its 3 outward inoperative position against the stop 26. A disk 29 of insulating material is provided between the end of the screw 8 and the end of the spring to electrically isolate the switch blade.

The circuit breakers are essentially normally closed single pole, single throw switches and when an instrument is equipped with either one or two circuit breakers they are connected in series with one of the internal instrument lead wires, thus the circuit through the instrument is open when either one of the circuit breakers is tripped.

In operation and. assuming an instrument is equipped with both left and right hand circuit breakers and both circuit breakers are in their normal position, as shown in Fig. 2, and in the event an overload is applied to the instrument of normal or reverse polarity, then the pointer 2 of the instrument will move beyond the scale and the pointer will impinge against abutment 12 of the blade 7 and move the blade into the Zone of attraction of the magnetic flux surrounding the ends of the pole pieces 15. Simultaneously, the circuit through the switch blade and the contact screw 11 will be broken and the blade temporarily retained against the pole pieces in the position shown, in dotted lines, Fig. 2, and illustrated by the right hand circuit breaker, Fig. 1. The movement of the blade will uncover the signal flag 27 and thus provide a visual indication that the circuit breaker is tripped.

To restore either circuit breakers to their normal position, the Zero-set screw 24 is first rotated 360. This will successively oscillate the restore levers 17 and restore blades 7 to their normal positions by virtue of the camming action of the zero-set levers 23 against the tabs 22. Following the restoration of both circuit breakers, the pointer may bev readjusted to its zero position by rotating the zero-set screw 24 relatively small angles in required direction without contacting tabs 22. A predetermined clearance is provided betwen the zero-set lever and the tabs 22..

An extremely important feature of the circuit breaker resides in the provision for sensitive interrelated adjustment of the tension of the switch blade supplied by screw 13, the width of the air gap 16 and the position of the contact screwfor establishing the range of movement of the blade 7.

In a preferred embodiment of the circuit breaker, it has been found that the switch blade 7 is satisfactory when made from spring steel .0015 inch thick and with a total mass in the order of 15 milligrams and plated with silver 0.0002" in thickness and the permanent magnet 14 having a, total flux of approximately 180 Gausses and the air gap 16 .025 inch in width. In order to provide sensitive diflerential adjustments required for various types of instruments, screw threads in the order of 80 per inch are satisfactory for tension screw 13 and screw contact 11.

By properly proportioning the differential adjustments previously described, the circuit breaker is applicable to electricinstmments having a wide range of siZe and type. For example, successful operation has been obtained with a pair of the above described circuit breakers in a two inch direct current -50 microammeter andthe same circuit breaker has been installed and adjusted to operate properly in a six inch direct current 01000 microammeter. By further precision adjustment to the circuit breakers, instruments may be protected for. as little as 50% overload and also for large overloads in excess of 50.0% without introducing suflicient ohmic resistance to alter the calibration of the instruments.

When the above described circuit breakers are used in instruments subject to unusual shock, such as in aircraft, it, has been found that improved stability of. the circuit breakeris obtained by constructing screw contact 11 with itshead portion a permanent magnet and the entire screw plated to provide proper conductivity. This alternate construction is preferred in some instruments instead of adjusting, the blade tension to a higher degree. By proper adjustment of the magnetic screw contact with respect to adjustment of the tension screw 13 the normal electrical contact thus established is highly resistant to shock without serious sacrifice in tripping sensitivity.

Having thus described my invention, I claim:

1. A circuit breaker attachment for a pointer type electric indicating instrument for interrupting its circuit comprising an elongated dielectric body adapted to be secured in said instrument, permanent magnet means including a pair of pole pieces of opposite polarity secured in said body, said pair of pole pieces projecting from one side of said body and forming an air gap of predetermined width and an eflective zone of magnetic flux in close proximity to said pole pieces, a flexible paramagnetic switch lever secured at one end to said body forming a sensitive flexible lever positioned for movement toward and away from said pole pieces, said lever normally biased toward said pole pieces, switch contact means adjustably secured in said body and positioned in the path of movement of the outer end portion of said lever, screw means in said body adapted to engage said lever and apply an opposing bias thereto for moving said lever into its normal position in contact with said contact means, an integral projection extending from said lever means adapted to be positioned in the path of movement of said pointer when said instrument is overloaded, whereby the overloaded movement of said pointer will impinge against said projection and move said lever from its contact with its said contact means into the attractive influence of said zone of magnetic flux and whereby said lever will be attracted into contact with said pole pieces and be held thereby, when said contact means and said screw means are in predetermined adjustment.

2. A circuit interrupting attachment for a poiriter type electric indicating instrument having. a Zero-set means comprising a dielectric body adapted to be secured in said instrument in the path of its pointer when said instrument is overloaded, permanent magnet means including a pair of pole pieces of opposite polarity secured in said body, said pair of pole pieces in one side of said body forming an air gap of predetermined width and an effective zone of magnetic flux in close proximity to said pole pieces, a flexible paramagnetic switch lever secured, at one end to said body forming a sensitive flexible lever positioned for movement toward and away from said. pole pieces, a portion of said lever positioned in the path of said pointer of said indicating instrument when the latter is overloaded, said lever normally biased toward said pole pieces, switch contact means adjustably secured" in said body and positioned in the path of movement of the outer end portion of said lever, screw means in said body adapted to engage said lever and apply an opposing bias thereto for moving said lever away from said pole pieces, into its normal position in contact with said contact means, a, restore lever pivotly secured to said body and having a portion thereof in the path of movement of said zero-set means and responsive to movement thereby, the opposite end of said restore lever adapted to abut and move said switchlever means from a position against said pole pieces to its normal position in contact with saidcontact means when: said Zero-set means is operated.

3. A circuit interrupting attachment for a pointer type electric indicating instrument comprising a dielectric body adapted to be secured in said instrument in the path of its pointer when said instrument is overloaded, permanent magnet means including a pair of pole pieces of opposite polarity secured in said body, said pair of pole pieces in one side of said body forming an air gap of. predetermined width and forming an eifective first zone of magnetic flux in close proximity tosaid pole pieces, a flexible paramagnetic switch lever secured at one end to said body forming a sensitive flexible lever positioned for movement toward and away from said pole pieces, a portion of said lever positioned in the. path ofsaidpcinter of said indicating instrument when the latter is overloaded, saidlever normally biasedtoward said pole pieces, switch contact means adjustably secured in said body in position in the path of movement of the outer end portion of said lever, said contact means including a permanent magnet for creating a SfiCOIZd zone of flux lesser in strength than the said pair of pole pieces, screw means in said body adapted to engage said lever and apply an opposing bias thereto for moving said lever away from said pole piece into its normal position in contact with said contact means and in the said second zone of magnetic flux, whereby the overloaded movement of said pointer will impinge against said switch lever and move 10 said lever from its contact with its said contact means and out of the said second zone of flux and into the attractive influence of said first zone of magnetic flux and whereby said lever will be attracted into contact with said pole pieces and held thereby when said contact means and said screw means are in predetermined adjustment.

References Cited in the file of this patent UNITED STATES PATENTS 692,217 Sundh Jan. 28, 1902 2,014,388 Lamb Sept. 17, 1935 2,302,250 Penn NOV. 17, 1942 2,543,630 Veevers et a1. Feb. 27, 1951 FOREIGN PATENTS 625,511 Great Britain June 29, 1949 

